The present invention relates to a controlling/driving apparatus for an electrically-driven compressor in a car.
Techniques for controlling/driving an electrically-driven compressor have been used mainly for domestic room air conditioners. In this case, the commercial power source which powers a room air conditioner has fluctuations in voltage of about .+-.5% of a nominal voltage and therefore the power source is quite stable. Accordingly, it has not been so necessary to take into consideration such fluctuations in power source voltage. However, in the case where an electrically-driven compressor is to be mounted in a car, no consideration has been taken on this matter either.
In a car air conditioner, however, if it is intended to drive a compressor by the power of a battery without relying on a conventional method in which the rotation of an engine is directly transmitted to the compressor, problems may occur as follows.
That is, the fluctuations in power source voltage in a car are very large in comparison with those of the domestic electric vehicle, the power source voltage falls when the car is accelerated because a large load current flows in the drive motor, while the power source voltage rises, due to a regenerative current supplied from the drive motor, when the car is decelerated. The range of such voltage fluctuations or variations can be larger than .+-.30%, which is very large in comparison to that of voltage variations in commercial power sources. Such a large range of voltage variations may be generated suddenly and frequently. FIG. 4 shows examples of such voltage variations in various running modes of an electric car. Accordingly, also the application voltage to an electrically-driven compressor fluctuates greatly in accordance with the running condition of the car.
In the case of an induction motor, therefore, the motor becomes overloaded (resulting in torque shortage) when the power source voltage falls, while it becomes overexcited (resulting in saturation of magnetic flux) when the power source voltage rises. Thus, the defects of increase of the motor current and lowering of the motor efficiency occur.
Particularly in the case of a high-pressure-hermetically-sealed-type electrically-driven compressor, which has an electrical motor portion installed in a high-temperature and high-pressure gas atmosphere, the temperature of the motor windings may easily exceed a tolerable temperature limit, which easily gives rise to a problem of deterioration of the electric insulation of the motor windings. A further problem is also involved such that once compressor operation is stopped because of overheating of the motor winding it can not be restarted until a pressure difference becomes lower than a given small pressure difference between the exhaust air pressure and the intake air pressure of the compressor. In certain examples it takes about two to three minutes from the stop to the restart of the compressor operation, and this gives rise to a further problem of temperature variations in the car compartment. In the case where a DC brushless motor is employed as the motor of an electrically-driven compressor, the speed of rotation becomes low when the power source voltage falls, and it becomes high when the power source voltage rises, so that there has been a problem that the speed of rotation of the compressor fluctuates greatly. As a consequence, the car air conditioner produces temperature variations at its exhaust port; or, referring to FIG. 5, compressing portion 11 of the air conditioner is subject to increased wear of portions on slide surfaces of the shaft 12 and bearing 13 due to such sudden variations in revolutions of the compressing portion, and such worn portions give rise to a problem of reducing the compressor durability.
The sudden variations in the revolutions in the compressor may result in discharging internal lubricant (oil) from the compressor, and in difficulty in recollecting such discharged lubricant resulting in shortening of the compressor's service life.
FIG. 6 shows a table indicating qualitative characteristics between revolution speeds, quantities of discharged lubricant and quantities of recollected lubricant with respect to a compressor. Significant problems may arise particularly in the case of sudden speed decrease to a low revolutional speed or in the case of a sudden speed increase in a high revolutional speed range. For the sake of comparison, it may be noted that the range of rotational speed variations in conventional electric compressors for home-room air-conditioners is on the order of about 5 revolutions/S.sup.2, and thus they have been free from the above-mentioned problems.
Car compressors of the above-mentioned type have another problem of possible increases in leakage currents flowing from the motor stator winding, through coolant, to the surface of the compressor with increases in the power source voltage, such increased leakage current is liable to lead to electric shocks being given to operators or users.